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07 April 2011 Solar Novus

Doped Quantum Dots Set To Improve Various Applications

Schematic illustration of doped nanocrystals, showing a single electron wavefunction from an impurity atom inside the nanocrystal (left) and multiple electrons from many impurities inside the nanocrystal (right)
Schematic illustration of doped nanocrystals, showing a single electron wavefunction from an impurity atom inside the nanocrystal (left) and multiple electrons from many impurities inside the nanocrystal (right).
Image Credit: Solar Novus.

Researchers at the Hebrew University of Jerusalem have successfully doped semiconductor nanocrystal quantum dots, thereby opening the way for improving quantum dot properties for applications such as solar cells.

Quantum dots have been resistant to doping because their small size causes the impurities to be expelled. Previous attempts at synthesizing doped nanocrystals have used high temperature techniques which probably exacerbates this problem, said Prof. Uri BaninProf. Uri Banin and his graduate student David Mocatta of the Hebrew University.Prof. Uri Banin and his graduate student David Mocatta of the Hebrew University. An additional challenge is that there aren't analytical techniques to study small amounts of dopants in the nanocrystals.

Banin and his graduate student David Mocatta of the Hebrew University Center for Nanoscience and Nanotechnology overcame these challenges by developing a simple room-temperature method to dope quantum dots with metal impurities. They could precisely control electronic properties, including the band gap and Fermi energy, by changing the dopant type and concentration. The method yielded n- and p-doped semiconductor nanocrystals that the researchers say greatly enhance the usefulness of such materials in solar cells, thin-film transistors, and optoelectronic devices.

Banin said that that they may be able to directly prepare nanocrystal p-n junctions from the solutions of n- and p-doped semiconductor nanocrystals, and because of the wet chemical processability of these nanocrystals, large areas of such p-n junctions may be fabricated in a facile manner. "This is a key challenge and central strong potential of using nanocrystals to produce devices we can employ a bottom-up approach of constructing the device from its components using chemical processing methods," he said.

In addition, because the quantum confinement effects are related to the tiny size of the nanocrystals, the absorption spectra of these nanocrystals can be altered by simply changing their size, Banin said. "In short, we hope that our discovery could help to produce large scale photovoltaic cells that cover a wide range of the spectrum in a cheap and simple way."

The doped quantum dots showed effects not previously reported. Bit by bit, in collaboration with Prof. Oded Millo of the Hebrew University and Guy Cohen and Prof. Eran Rabani of Tel Aviv University, the researchers built up a comprehensive picture of how the impurities affect the properties of nanocrystals. The initial difficulty in explaining this process proved to be a great opportunity, as they discovered that the impurity affects the nanocrystal in unexpected ways, resulting in new and intriguing physics.

"We had to use a combination of many techniques that when taken together make it obvious that we managed to dope the nanocrystals. It took five years but we got there in the end," said Mocatta. This work sets the stage for the development of potential applications with nanocrystals, ranging from electronics to optics, from sensing to alternative energy solutions. Doped nanocrystals can be used to make new types of nanolasers, solar cells, sensors and transistors.

So far the researchers have successfully doped only InAs from the III-V family. "The doping of other semiconductor types and using different impurities is one of the open questions," said Banin. "In addition, the physics of these systems is not straight forward and requires further treatment. We are building on our discovery and working on these questions."

Source: Solar Novus /...

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